Nick Clegg predicted that the UK will face another General Election before Christmas if the Liberal Democrats do not form part of a coalition government following this week's poll.
The Lib Dem leader said it would be impossible for a minority government to pass key legislation without signing off controversial concessions to the SNP and Ukip.
A party statement said a second election in 2015 would be "almost inevitable" without the Lib Dems in parliament.
Everybody knows that no one will win this election – even if David Cameron and Ed Miliband won’t admit it publicly.
If they try to stagger through with a messy and unstable minority government instead of putting the country first then they will risk all the hard work and sacrifices people have made over the last five years.
The last thing Britain needs is a second election before Christmas. But that is exactly what will happen if Ed Miliband and David Cameron put their own political interest ahead of the national interest.
The only party that will ensure stability is the Liberal Democrats.
The SNP leader Nicola Sturgeon has said it would be undemocratic and insulting to Scottish voters - if her party wins the most seats there, but has no voice in the next Parliament. She told voters in Livingston they had 48 hours to get the Tories out.
David Cameron has said anyone in a Liberal Democrat-held seat should vote Conservative to avoid the prospect of a Labour-SNP government.
Asked whether he will need Nick Clegg to form a government after May 7, the prime minister told ITV News' James Mates: "I have a clear message for anyone in a Liberal Democrat held seat and that is vote Conservative.
"If you vote Liberal Democrat you don't know what you are going to get. Nick Clegg was very clear at the weekend he's just as likely to support an Ed Miliband-SNP government - that would bring the country to a juddering halt - than he is to support the Conservatives."
Boris Johnson says he has never "been so worried" about the prospect of Labour returning to power.
"Never in my life - this is the sixth election I have fought - never have I been so worried about what could happen under Labour," Mr Johnson, speaking alongside David Cameron, said in Hendon.
The Mayor of London said Mr Miliband had "junked sensible centrism under Tony Blair" in favour of an anti-business message which would drive investment and jobs away from the UK.
He also warned of what he called the "chaos" of a Labour government working with the SNP, claiming Alex Salmond would have "his feet up on Miliband's desk in Downing Street holding out his glass for more pink champagne paid for by the English taxpayer".
Ed Miliband has told voters there are "less than 48 hours to get rid of this Tory government and David Cameron" as he placed the NHS at the centre of his campaign again.
Mr Miliband told an audience in Bedford that the health service was facing a "financial timebomb" with the Tories planning "savage cuts".
He said he wanted to put the "right values" including "care, compassion and corporation" back at the centre of the NHS.
The Labour leader said he would keep going "right till the last minutes" to rescue the National Health Service and prevent another five years of Conservative government.
ITV News' Carl Dinnen, who is following Labour's campaign, said Mr Miliband had admitted "this race could be one of the closest in our history" as he outlined the choice facing voters on Thursday.
David Cameron has compared himself to a "firefighter" and Ed Miliband to an "arsonist" as he outlined his vision ahead of Thursday's election.
"I feel like the firefighter, hosing down the burning building, and there's Ed Miliband - the arsonist - saying 'why aren't you doing it quicker?'," Mr Cameron said at an event in Twickenham, south-west London.
Brandishing the infamous note left by Labour's Liam Byrne, Mr Cameron said "everything" at this election "comes back to the economy".
Mr Cameron was briefly interrupted by a heckler during his speech but the prime Minister said: "You need to listen to this sir, it's important - if you don't want chaos you need to get out and vote Conservative."
Theresa May insists the Conservatives can still win a majority at next Thursday's election.
Despite polls predicting another hung parliament, Mrs May refused to discuss the prospect of any post-election deal, saying the Tories were still focused on winning the 23 seats needed to form a majority government.
"We just need 23 seats in order to be able to get that - that would enable us to be able to have the stability to secure the country and people's future," the home secretary told the BBC.
"The alternative on Thursday is for people to vote for a Labour government propped up by the SNP."
She said a Labour-SNP government would mean "higher taxes, more borrowing, more spending on welfare" and further debt.
The Liberal Democrats will work with the party with the biggest mandate in the event of a hung parliament, Nick Clegg has said.
However, Mr Clegg did not specify whether this meant the party with the most number of seats or the largest share of the votes.
Asked whether he preferred to work with Labour or the Tories, Mr Clegg told BBC Radio 4: "The party which gets the biggest mandate...[it] seems to me right to give them the space and time to try and form a government."
He accused David Cameron and Nick Clegg of "preposterously charging around the country saying they're going to win an outright majority", adding that the Lib Dems want a "stable, decent and united government" after Thursday's vote.
Nick Clegg said his "epic journey" across the UK shows how much he "cares" about the country.
As he embarks on a 1,000-mile trip ahead of Thursday's election, the Liberal Democrat leader said: "It's just an illustration of how much I care for our wonderful country and want to communicate to as many people as I can that we need to remain stable and decent and united as a country, not lurch this way or that."
Amid mounting speculation about potential post-election deals, Mr Clegg insisted he had not engaged in talks with either the Conservatives or Labour, saying he was waiting for the "judgement of the British people".
Mr Clegg was boosted by a poll last night which suggested he was on course to retain his Sheffield Hallam seat despite a strong effort by Labour.
Political leaders have taken to the road in a final bid to gain voters as election day draws nearRead the full story ›